A known image forming apparatus has a function to, for example, correct deviation in an image forming position on a sheet. Specifically, in the correction function, a pattern (such as a registration pattern) configured by a plurality of marks is formed on a belt. Next, on a basis of signal waves contained in a light reception signal from an optical sensor having a detection area on the belt, the positions of the marks corresponding to the signal waves are detected. On a basis of a result of the detection, the deviation in the image forming position is corrected.
However, a factors such as a response delay of the optical sensor to the time when the mark passes the detection area can cause errors between the mark positions detected based on the signal wave and respective actual position. This error can decrease accuracy in correcting the image forming position. Therefore, there is an art addressed to reduce the errors between the detected mark position and the actual position by stepwisely increasing and decreasing an amount of toner attached in each of the marks.
However, in the art, a process of increasing and decreasing the amount of toner attached in the mark is necessary, which is troublesome. Therefore, there is a need for another means. Furthermore, even where the above-described art is adopted, the errors between the detected mark position and the actual position can sometimes still be caused. Therefore, there is a need for further improvement.